Rheostat



H. L. DODGE.

RHEOS TAT.

APPLICATION FILED AUG-23, I917. RENEWED MAR. 4, I920- I 1,337,826. Patented Apr. 20, 1920, F I 1 2 SHEETS-SHEET 1.

1 W a 9 2 r WITNESSES ATTORNEY L. DODGE. RHEOSTAT.

APPLICATION FILED AUG-23,,I9IZ. RENEWED MAR. 4, I920.

Patented Apr. 20, 1920.

2 SHEETS-SHEET 2.

WITN ESSES Mam/m ATTORNEY UNITED STATES PATENT orrion HoMnn L. DODGE, or Iowa. CITY, IOWA.

RHEOSTAT.

menses.

Specification of Letters Patent. Patented Apr, 20, 1920.

Application filedAugust 23, 1817, Serial No. 187,820. Renewed March 4,1920. Serial No. 363,288.

To all whom it may concern Be it known that I, Honrnn L. DODGE, a citizen of the United States, residing at Iowa City, in the county ofJohnson and State of Iowa, have invented a new and useful Bheostat, of which the following is a specification.

This invention has reference to rheostats embodying the principle'of and also constituting an improvement over the rheostat shown, described and claimed in Patent No. 1,195,660, granted to me on August22, 1916. The object of the invention is to provide a construction working on the same general principleas the rheostat shown in said patent, but which may be of large capacity and includes a number of separate resistance elements, with the construction such that the cost due to wastage of current because of constant HOW to a large degree through the rheostat, and losses due to unnecessary or undesirable heating, are obviated.

I he invention comprises means for the purposes named and will be best understood from a consideration of the following detailed description, taken in connection with the accompanying drawings forming part of this specification, with the further under standing that while the drawings show a practical form of the invention, the latter is not confined to anystrict conformity with the showing of the drawings, but may be changed and modified so long as such changes and modifications come within the scope of the appended claims.

In the drawings Figure 1 is a diagrammatic representation of a large capacity rheostat embodying the invention.

Fig. 2 is a diagrammatic representation of a similar rheostat embodying certain other features.

Largeaca pacity rheostats may, for mechanical reasons, be made up of a suitable number or" separate resistance elements, especially in cases where there'sistance wire employed is wound on tubes. In such construction it is customary to employa steel tube covered with porcelain in order to give a suitable insulating surface, and then resistance wire is wound upon the porcelain surface of the tube from end to end. Usually such wire has an oXid surface which insulates the turns from each other. Furthermore, a slider is provided to make contact with any part of the winding, the wire,

of course, being freed from oxid where necessary to provide good contact between the slider and the-wir convenience of description the porcelain covered steel tube with resistance wire wound thereon will be hereinafter simply referred to as a tube.

h'ioreover, it may also be assumed for convenience of description that the tubes are arranged. in a group of two or more and stand upright with the slider moving up and down the group of tubes. Of course, this does not preclude other arrangements, but will serve to simplify the description, especially where terms ofdirection of movements are used.

When a rheostat is made up from two or more tubes it is possible, by certain peculiarities in construction, to avoid waste of electrical energy or undesirable heating. T he tubes are so arranged and connected that the second and subsequent tubes of a multitube rheostat may be thrown in parallel with the first, one at a time, by means of an appropriate switch, but in order that each tube 1 may increase the current capacity of the rheostat by as great an amount as possible, and yet not afford a possibility for being burned out, or heated excessively, the winding of each tube must be arranged in a particular way.

F or an understanding of the invention it may be assumed that a rheostat is made up of four elements A, B, C, D, although, of course, a greater or less number of elements may be employed. It may be further assumcd that the elements are made up of resistance wire wound upon suitable supports, the supports and wire constituting what has hereinbetore been termed tubes. The length of the tubes and the size of the wire will depend upon circumstances, but by way of example it may be assumed thatthe tubes are twenty-two inches long and two inches in diameter wound with various sizes of wire in different lengths, as will be hereinafter referred to.

The rheostat is provided with line receiving terminals 1, 2, respectively, and with load receiving terminals 3, l, respectively,

which latter are connected to respective sides of a two pole switch 5 capable of movement into engagement with other terminals 6, 7, respectively.

In the particular showing of Fig. 1 the line terminal 1 is connected by a conductor 8 to the switch terminal 6, while the line ter minal 2 is connected by a conductor 9 to one end of the resistance element A, and for convenience of description it may be considered that the switch elements. are arranged in an upright position with the conductor 9 connected to the top or upper end of the resistance element or. tube A. The other end of the tube A is connected by a conductor 10 to a contact terminal 11 adj acent to but insulated from which is another contact element 12 connected by a conductor 13 to the switch terminal 6. The line terminal 2 is connected to another conductor l-cl leading to a terminal 15 in the path of another switch 16 capable or engaging other terminals 17, 18 and 19 one at a time in order, and all at the same time, in a manner and for a purpose which will hereinafter appear. The contact terminal 17 is connected by a conductor 20 to the upper end of the resistance element or tube B. The contact terminal 18 is connected by a conductor 21 to another resistance element or tube C which in turn is connected by a conductor 21 to the upper end of the resistance element or tube C. The conductor 19 is connected to one end of a resistan e element or tube D, the other end of which is connected to the upper end of the resistance element or tube D.

There is also provided a slider 22 common to all the resistance elements A. to l and capable of movement toward and from the upper ends of these elements simultaneously, while making contact with the windings thereof. The slider is connected by a conductor 23 to the switch contact 7 and this conductor may include an ammeter 24, while branched from the ammeter is another conductor 25 including a voltmeter 26 and connected to the terminal 6 of the switch 5. In the event that the ammeter and voltmeter are not included in circuit with the rheostat the conductor 25 is omitted, and the conductor 28 is carried to the switch terminal 7 without including an ammeter in circuit with it.

The switch 16 is shown as of plate form capable of bridging certain of the contacts 11, 12, 15, 17, 18 and 19, and is to be taken as indicative of any suitable switch arrangement which will perform the functions of: the particular type of switch shown. For convenience, the switch 16 may be manipulated by means of a knob 26", and is further shown as provided with a notched disk or wheel 27 into any one oi? the notches of which a tooth 28 on one end of a lever 29 may engage, the other end 30 of the lever being in the path of the slider 22 to be engaged thereby when the slider is at its lowermost position, so that all the resistance elements are wholly included between the slider and source when the switch is released for movement. It will be understood, however, that these features while valuable for some purposes are not essential for other purposes and may be employed or discarded in ditlerent embodiments of the invention.

Iii it be considered that the load across the terminals'izl and 1 is oi? zero resistance, then as the slider is moved toward. the upper or intake ends of the resistance elements the current becomes correspondingly greater, and hence each resistance element must include between the highest point of movement of the slider and the intake end ol the resistance element enough wire of the chosen gage to limit the current to a point where it will not overheat the wire. Furthermore, to obtain a desirable gradal ion of resistance within desirable lengths ol tubes, each tube may be wound with succes sively smaller gages of wire from the receiving end toward the other end of the tube. This means that there must be a sufficient length of wire from the receiving end of the tube to the limit of movement of the slider toward the receiving end of the tube to protect the wire there located so as to prevent overheating. Moreover, such length of wire must also be long enough to protect the wire of next smaller gage, and so on. l urthermore, account must be taken of the fact that the tubes are to be utilized in multiple or parallel, and this also affects the winding both with respect to the gage of wire and the length thereof.

The invention will be best understood from a consideration of a practical embodiment worked out to meet certain conditions, as for instance it may be assumed that the feed circuit is one supplying current at 110 volts, and each tube A to D is twenty-two inches long and two inches in diameter and is capable of dissipating heat at the rate of five watts per square inch.

In considering overheating of the wire on the tubes and the length of wire necessary to protect itself and the wire of next smaller gage, it is to be understood that the amount of current which a given winding may carry depends upon the rate at which heat can be dissipated from the element, and this in turn depends upon the temperature to which the resistance elements may be permitted to rise, the ventilation, and other conditions. The test of overheating would, therefore, depend upon whether or not an arbitrary limit, chosen as being the most desirable in a particular instance, were exceeded.

Practical conditions of construction'must be taken into consideration, such for instance as the fact that commercial wire varies in size by appreciable amounts wherefore different resistance elements may have values of maximumwattage dissipation varying from an assumed normal value.

In the exampleto follow simplicity of description forbids taking account of such variations as must be considered by a manufacturer, but it is to be understood that the invention covers variations that must be taken into account both in design and manufacture. Now, let it be assumed that tube A has its upper or receiving end wound with #18 resistance wire of a particular kind. Furthermore, let it be assumed that it takes as much of #18 resistance wire as will occupy sixteen inches in length of the tube when closely wound to protect the next smaller size of wire from overheating by the amount of current which will pass the sixteen inches of winding of #18 wire at 110 volts. However, #18 wire takes but twelve and four-tenths inches of winding lengthwise of the tube to protect itself against overheating. Such wire has a resistance of two and one-tenth ohms per inch of winding, and consequently at 110 volts will carry four and two tenths amperesof current. This is, therefore, the maximum current which tube A can carry. The next smaller wire is #19, and hence after #18 wire has been placed on the tube A for a length of sixteen inches thereof, then the winding may be continued in #19 wire. Only one inch of #19 wire in'addition to 16 inches of #18 wire is necessary to protect wire, but in order to keep the resistance from getting too large in a twenty-two inch tube,

a coil three inches long of #19 wire is placed on the tube, and this is followed by a coil three inches long of #20 wire. There is therefore wound on the tube a coil twenty-two inches long made up of #18, #19 and #20 wire with a resistance of thirty-three ohms for sixteen inches of #18, a resistance of ten ohms for three inches of #19 wire, and a resistance of twelve ohms for three inches of #20 wire, these figures very closely approximating the actual conditions. The total resistance of tube A is, therefore, about fifty-five ohms, and the tube will give a maximum current of 4.2 amperes and by the application of the shunt-series principle set forth in the aforesaid patent will give any. current down to zero.

Since the maximum current that tube A can carry is 4.2 amperes under the conditions assumed, tubes A and B in parallel must have sufficient resistance to give as small a current as 4.2 amperes in order that the ranges may overlap. Now, to give a current of 4.2 amperes at 110 volts, the r sistance must be about twenty-six ohms, and this must be the total resistance of tubes A and B in parallel. l l ithout going into the calculation it may be stated that tube B may be wound with wires #17, #18 and #19 of which seventeen inches of #17 is sufficient to protect #18, and a small additional amount of #18 is sufiicient to protect #19, while the total resistance of the three wires in the winding in series equals fortyseven ohms, and the total resistance of the winding on tube A equals fifty-five ohms. The minimum current of tube A is therefore two amperes,'while that of tube B is 2.3 amperes, wherefore the minimum current of tubes A and B in parallel is 4.3 amperes, which is sufliciently close to the maximiun of tube A alone, namely, 4.2 amperes. Now, it takes a coil of #17 wire fourteen inches long on a two inch tube to protect itself, wherefore at zero resistance of load the slider must not be moved up higher tl an fourteen inches from the top of tube B. The maximum current in tube A when the slider is at fourteen inches from the top is 8.8 amperes and for tube B is 4.7 amperes, so that 8.5 amperes is the maximum for tubes A and B in parallel.

New in order that the capacity of tubes A, B and C in parallel may overlap that of tubes A and B in parallel, tube C must have such a resistance as to permit of. a minimum current with tubes A, B and C, in parallel at the minimum point. that will overlap the maximum for tubes A and B in parallel. The resistance of tube (l is found on calculation to be about twenty-six ohms. t is desirable to use as large wire as possible, but the troulde encoun ered that it takes more than fourteen inches length of coil on a two inch tube of any of the larger sizes to protect itself. This would mean that the slider :annot be raised to the fourteen nch point, and consequently there would necessarily be a lower maximum rating for all tubes, poorer overlapping, etc. To overcome this difficulty a portion of the larger wire may be wound upon a separate or supplemental tube out of the path of the slider, so that the slider may be raised to the fourteen inch height on all three tubes. Now for tube (l the larger wire may be of gage, taking twenty inches length of coil to protect itself. and twenty-two inches length of coil to protect #16, which is the next smaller gage. This means that an eight inch coil of #15 wire is wound on tube and fourteen inches of #15 wire is wound on tube making in all nineteen ohms, and then this may be followed by eight inches of #11) vire having a resist ance of nine ohms. wherefore the resistance of tube U with its supplemental tube U totals twentyeight ohms. giving a mini mum current of about four amperes which added to the minimum for tubes It and B in pai-Jallel or 4.3 amperes, gives 8.23 ampores, overlapping the maximum for tubes A and B.

lVith the slider at fourteen inches the maximum current for tube C is 5.8 amperes, and this added to the maximum of tubes A and B in parallel gives 143 amperes.

In a similar way it is found that the total resistance of tube D should be about seventeen ohms and with this it is possible to use #13 wire of which thirty-two inches is required to protect #14, the next smaller gage. A portion of this wire is wound on tube D and the remainder on tube D, and then a smaller amount of #14 wire is continued on the lower end of tube D, giving a sufficient resistance, namely, 15.2 ohms to approximate the theoretically required seventeen ohms. The minimum current for tube D is 7.2 ohms, which with the minimum of tubes A, B and C gives 15.5 amperes for the minimum of the four tubes in parallel, which is a sufficiently close approximation to the maximum of tubes A, B, and C. The maximum of tube 1) is 9.5 amperes so that the maximum of all four tubes is 23.8 amperes.

In the foregoing examples the assumption of a certain wattage capacity for each tube, and consequently a certain current capacity for each size of wire, is arbitrary. It is not always desirable that the results of theory be approximated too closely. From the examples given it will be seen that complete overlapping of range is not in every case secured with a load of zero resistance. The necessary overlapping may be secured by a slight overloading of the smaller number of tubes beyond the assumed limit. If the load had more than a certain amount of resistance then the ranges would overlap without overloading. For instance the practical embodiment might be worked out with the assumption of a load of, say, 5 ohms resistance. The windings would then, for the best results, have a slightly different distribution from that described.

In the example given the load has been assumed as one of zero resistance, not because it is a usual case but because it is the worst case.

The slider makes contact with all four tubes at once and at all. times in the arrangement shown in Fig. 1, but provision is made by means of the switch 16 or by some other similar means so that any one or any group of the tubes may be included in the circuit.

By means of the switch 16 and the con tacts 11 and 12, tube A may be made to pass current within its range or with the contacts 1.1 and 12 bridged by the switch and the slider at some intermediate point give various currents on the load side as described in the aforesaid Letters Patent,

With the switch 16 so arranged as to bridge the contacts 15 and 17 only, and the slider at any chosen point, the two tubes A and B are in parallel arrangement and the current delivered to the load side of the rheostat corresponds to the position of the slider with res met to the two tubes A and B in parallel. Vhen the switch 16 is moved so as to include the contacts 15, 17 and 18, then tubes A, B and C are in parallel, and when the switch 16 is moved to include the contacts 15, 17, 18 and 19, then all four tubes are in parallel, the position of the slider determining the total current delivered up to the maximum which is obtained when the slider is, in the particular arrangement of the drawings, fourteen inches from the upper ends of the tubes.

The lever 29 is designed to hold the switch 16 from manual operation except when the, slider 22 is at its lowermost position, that is, at the position farthest from the leading-in ends of the tubes. When the slider is moved to include the entire lengths of the tubes, then it engages the end 30 of the lever 29 and lifts the nose 28 from the notch in the wheel 27 in which it may at the time engage, thus permitting such ad'ustment of the switch 16 as may be desired In Fig. 2 the arrangement is the same as in Fig. 1 except for the addition of some features somewhat enlarging the scope of the rheostat. The switch plate 16 carries an insulated bridging-piece 27 in the path of which are two contacts 28 29*, one of which is connected by a conductor 30 to the entering end of one of the tubes, say, the tube C,

and the other of which is connected by a conductor 10 to the lower end of tube A. The slider has a contact member 31 on but insulated from the slider, for contact with the tube A, while a switch 32 on the slider permits electrical connection of the slider with the contact 31, whereby the circuit between the slider and the tube A may be established or broken as desired. This arrangement may be provided for others of the tubes, but the action of this feature of the invention can be set forth with relation to one tube so that it can be well understood with reference to any one or more of the tubes. With the switch 16 moved to a sufficient extent to bridge the contacts 28. and 29 by the bridging member 27 and the switch 32 open, the circuit may be traced from the terminal 1 through the conductor 8 to the load terminal 3, thence back by way of the load terminal 4, conductor 23 to the slider, thence by tube C, conductor 30, bridging plate 27, conductor 10 to and through the entire length of the tube A to the other line terminal 2. This permits very small variations in the resistance, and these variations are more uniform than as shown in Fig. 1, since the resistance steps between the turns of large wire are of much smaller value than in the case of the smaller wire, while the current delivered is in itself small because of the constant inclusion of the tube of highest resistance.

While sizes of wire and lengths of coils of wire have been indicated in the drawings, it is to be understood that this is done merely by way of example, although substantially correct for a certain condition. For other conditions sizes and lengths will differ from those indicated, but will follow the same general principle.

lVhile a certain arrangement has been indicated with the resistance units in parallel, so that with a single slider the loading of each unit shall be a maximum at the same position of the slider, with Zero resistance load, the result may be obtained with other constructions than the particular one described. Instead of employing tapering windings as indicated, a similar result might be had by spacing or crowding the wire or making the wires of different materials or by the use of different sized tubes or wires of other shapes than round in crosssection.

It has been hereinbefore stated that the slider is limited in its upward movement, that is, toward the entering ends of the tubes, to a point 14 inches from the top of the tubes, or, of course, this distance may be greater or less with a different rheostat from the particular one under consideration' This arrangement is under the assumption that the load is of zero resistance. If the load has an appreciable resistance then the slider has to be raised higher, or closer, to the entering ends of the tubes in order to secure the desired current.

It is to be further understood that it is not obligatory to employ a slider movable to an equal distance over the various tubes since other arrangements are feasible.

Nor is it necessary that the slider, or other contact maker be movable into direct engagement with the wire of the tubes, since it is quite feasible to take out leads from the tubes to suitably arranged contacts and move the slider or other contact maker accordingly. The term slider, therefor, is to many variations readily apparent to those skilled in the manufacture and use of rheostats.

No showing has been made for any casing for the rheostat, but as it is common to incase rheostats it will be understood that a suitable casing may be provided if deemed necessary.

Instead of using tapered windings, or spacing or crowding the wires, or employing other expedients hereinbefore named, the units may be so constructed that they will change their radiating surfaces by different amounts for the same motion of the slider.

What is claimed is 1. A rheostat comprising a plurality of associated resistance elements, a slider common to the resistance elements, and means in addition to the slider for including one or more of the resistance elements in circuit.

2. A rheostat comprising a plurality of associated resistance units or elements of different capacities, a slider common to the resistance units and operable at will, and means for connecting the windings of the resistance elements in different relations, including connecting them in parallel and also operable at will.

3. A rheostat comprising a plurality of associated resistance elements, means for simultaneously including portions of the resistance elements in circuit, and means for throwing the resistance elements into or out of parallel relation with each other, the resistance elements having their resistances arranged in connection with the means for including them in the circuit to provide values of current and voltage from Zero to a predetermined maximum.

4:. A rheostat composed of a pl rality of resistance elements, means for throwing the elements into and out of parallel relation with each other, and a slider common to the resistance elements, the resistance of the diil'erentelements being related to obtain a maximum current from a certain number of elements as great as the minimum current from the next larger number of elements.

5. A rheostat composed of a plurality of J..

associated resistance units or elements having the resistance of each differing from those of the others with the resistance of any number of the elements connected in parallel greater than the minimum resist ance required to permit operation without overload of the next lower number of elements.

6. Arheostat comprising units composed of a plurality of serially connected wires of different gages with the amount of wire protective of the next winding of smaller gage.

7. A rheostat having resistance units each composed of a plurality of serially connected windings of progressively decreasing gage from the entering end of the resistance element, with the amount of wire in the first winding in order of a length protective of itself against undue rise in temperature, and also protective of the wire of the next smaller gage in order.

8. A rheostat having a plurality of associated resistance units and a slider common to said resistance units, the gage of the wire difl'ering on the different resistance units and the amount of wire between the slider and the entering ends 01 the units being such as to permit the maximum permissible current to pass with the slider at the same distance from the entering ends of all of the units engaged by the slider.

9. A rheostat comprising a plurality of resistance units or elements of successively increasing carrying capacity, a slider common and movable to equal extents along all the units, and means for coupling up the units in parallel relation.

10. A rheostat comprising a plurality of resistance units or elements of successively increasing carrying capacity, a slider common to all the units and movable equally therealong, and means for coupling up the units in parallel relation, the resistance of the elements being progressively less in order by amounts to cause a lower number of units to deliver a greater maximum current than the minimum current delivered by the next higher number of units.

ll A rheostat comprising a plurality of resistance elements of progressively increasing carrying capacity, a slider common to all the elements thereof and movable simultaneously therealong, means for coupling a plurality of elements in parallel, and means for connecting a chosen number of elements to varying extents across the line to which the rheostat is connected.

12. A rheostat comprising a plurality of associated resistance elements, a slider com-- mon to all the elements and'movable therealong to equal extents, means for connecting a chosen number of the resistance elements in parallel relation, means for including a chosen number of resistance elements to chosen extents in shunt with the line in which the rheostat is included, means for including a chosen amount of a resistance element of large carrying capacity with the entire extent of a resistance element of smaller carrying capacity, and means for connecting or disconnecting the element of smaller carrying capacity with respect to the slider, whereby the rheostat may be adjusted by smaller steps than when including a resistance element of small carrying capacity.

13. A rheostat comprising a plurality of resistance elements, a slider common to and movable at will with respect to the resistance elements to vary the current through the rheostat, switch means for coupling the resistance elements at will to desired extents in parallel, and locking means for the switch means in the path of the slider to release the switch means when the slider is in position most distant from the current entering ends of the resistance elements.

let. A rheostat comprising a plurality of resistance elements of common length and respectively different carrying capacities and including resistance wires of different gages, a slider common to all the resistance elements and movable therealong with the length of resistance wire at the entering ends of the different elements sufficient to be self protective against undue heating under the current employed and to protect the next smaller wire in order, and supplemental resistance elements constituting adjuncts to those provided with the larger gages of Wire to make the length of wire used sufficient to be self protective without prolonging the length of the elements with which the slider is associated.

15. A rheostat having a plurality of associated resistance tubes with certain of the tubes each having an auxiliary tube connected in series with the entering end thereof, and a slider common to the resistance tubes.

16. A rheostat having a plurality of associated resistance tubes, with the gage of wire differing on the different tubes, and certain of those tubes carrying Wire of larger gage having auxiliary tubes connected in series with the entering ends thereof, and a slider common to the firstnamed tubes.

In testimony that I claim the foregoing as my own I have hereto afiixed my signature in the presence of two witnesses.

HOMER L. DODGE.

lVitnesses A. B. GILBERT, F. W. KENT. 

